Research has been conducted on an electrophotographic photoreceptor containing organic photoconductive materials because it offers several advantages such as no environmental pollution, higher productivity, and lower costs. There is a prior art electrophotographic photoreceptor which comprises a photosensitive layer containing diphenyldicyanoethylene derivatives as a sensitizer. Such an electrophotographic photoreceptor is disclosed in, for example, JP-A-54-30834 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
A problem with such use of organic photoconductive materials is present that a material which absorbs visible light to generate a charge exhibits poor charge retaining capacity; in contrast, a material which exhibits good charge retaining capacity and excellent film forming properties barely exhibits photoconductive properties by visible light.
To overcome such problems, use of a photosensitive layer of a laminate type composed of a layer containing a charge generating agent which absorbs visible light to generate charges and another layer containing a charge transporting agent to transport the charges produced has been attempted. Various charge generating agents and charge transporting agents have been suggested. For example, when the charge transporting agent is the hole transporting agent type, amine compounds, hydrazone compounds, pyrazoline compounds, oxazole compounds, oxadiazole compounds, stilbene compounds, or carbazole compounds may be used. Also, it is known that 2,4,7-trinitrofluorenone is an electron transporting agent type in the charge transporting agent. In addition, JP-B-48-9988 (the term "JP-B" as used herein means an "examined Japanese patent publication") and Canadian Patent 912019 disclose a compound containing boron as a charge transporting agent.
In prior art electrophotographic photoreceptors comprising a single photosensitive layer containing organic photoconductive materials, a completely practical sensitizer has not been found.
In a function separated type electrophotographic photoreceptor comprising laminated photosensitive layers, it is preferred that a positive charged type be used because the generation of ozone in Corotron is prevented, and toner charges are controlled upon development. When an electrophotographic photoreceptor is of the positive charged type, and when a charge transporting agent is a hole transporting agent, it is necessary to use a charge generating layer as an upper layer. However, because the custom is to make the charge generating layer thin, the problem arises that such a thin charge generating layer is insufficient to meet the mechanical requirements of an electrophotographic photoreceptor. Also, it would be commercially advantageous to improve the properties of a positive charged type electrophotographic photoreceptor because otherwise it would be necessary to devise a copying machine capable of using a negative charged one.
On the other hand, there is a need for an electrophotographic photoreceptor of a positive charged type in which a considerably thick charge transporting layer constitutes the upper layer. To this end, it is necessary to use a charge transporting layer containing charge transporting agent having an electron transporting property. However, among conventional charge transporting agents having an electron transporting property, there are not known those which are fully practical.